SDS-concentration-dependent α-synuclein structure: Upon interaction with SDS, Syn folds into a structure with two antiparallel α-helices. We show from single-molecule FRET that Synn adopts this conformation in an all-or-none fashion below the SDS critical micelle concentration. Population of the folded species is directly coupled to an increase in α-helix content; this suggests that the entire N terminus is involved in the transaction.